CHANGES IN SUPRATIDAL COASTAL BOULDER DEPOSITS (ARAN ISLANDS, IRELAND) MEASURED USING STRUCTURE-FROM-MOTION PHOTOGRAMMETRY

Supratidal coastal boulder deposits (CBD) incorporate clasts with masses 10s to 100s of tonnes. It takes very large waves to activate these deposits, which makes them archives of high-energy events; but their dynamics are difficult to study, as many years can pass without major changes. A series of very strong storms in the winter of 2013-2014 brought an unusual opportunity to examine the before-and-after states of CBD in western Ireland. In the aftermath, we have been documenting the effects of the storm waves, which moved existing boulders, created new clasts from bedrock, and caused inland migration of some CBD.

To measure the changes, we flew an unmanned aerial vehicle (UAV) survey of coastal boulder deposits on Inishmore, one of the Aran Islands off Ireland’s west coast. We used the structure-from-motion (SfM) software package Agisoft PhotoScan Pro to construct high-resolution 3D models of the coastal topography and boulder deposits. We compared the SfM models—which characterise post-storm disposition of the CBD—with pre-existing orthoimagery and with the Geological Survey of Ireland’s 2003 coastal oblique aerial survey images. The comparisons reveal changes in the morphology of boulder ridges, reorganisation of boulder clusters, and relocation of individual large clasts.

The energy distribution on the coastal platform, as recorded by which boulders moved and which did not, was highly variable. In some cases very large clasts were moved considerable distances, while smaller clasts nearby moved not at all. Seaward faces of some boulder ridges were completely rearranged, but in others only a few of the constituent clasts changed location. At some sites there was extensive landward migration of clasts (from the platform onto the boulder ridge face, from the ridge base to the crest, and from the ridge crest into the back-ridge area), whereas at other nearby ridge sections there was far less change.

Initial comparisons of pre-and-post storm boulder dispositions used terrestrial rephotography of field sites, and we used those before-and-after images to ground-truth the SfM-orthoimagery comparisons for specific locations. However, the UAV survey provided far more comprehensive image coverage of the entire CBD system, permitting us to construct generalised, regional assessments of the recent changes.